The basic operating principle of a microwave barrier is known e.g. from DE 33 02 731 A1. A transmitter emits a continuous-wave microwave signal in a direction of an associated receiver. If the direct transmission path from the transmitter to the receiver is interrupted by a target object or by a filling level in a container, this will be recognizable by a missing or a reduced receiving level in the receiver. The decrease of the receiving level is detected within the receiver and is converted into a switching signal. With such a simple arrangement it cannot be prevented e.g. that due to pollution of the microwave transmitter or the microwave receiver a physically not existing target object is detected falsely, because also by pollutions the receiving level, meaning the amplitude of the received microwave signal, can sink. Particularly in industrial applications an accordingly pollution might become unavoidable, e.g. due to dust deposits in machining applications or due to soot deposits in applications with smoke development, by which sometimes considerable process malfunctions are caused.
To avoid such a false detection (“false positive”) it is possible e.g. to maintain microwave transmitters and microwave receivers periodically. For example a microwave antenna can be examined by professional personal with a focus on pollution and can be cleaned, if necessary. Also, an automatic cleaning is possible, which may include the use of monitoring means such as cameras, which detect and signalize pollution or other deposits in the area of the microwave transmitter of the microwave receiver.
However, this solution requires additional efforts, either in form of personal or in form of technical devices for detecting and removing of pollutions. Also, both possibilities additionally generate further potential sources of errors. Therefore, a microwave barrier for monitoring a limit level would be desirable, on which pollutions do not have an effect.